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[tip:locking/urgent] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"

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Commit-ID:  f17b3ea3d2df7c9bf3ce1dbd65b5fd7061f8e787
Gitweb:     http://git.kernel.org/tip/f17b3ea3d2df7c9bf3ce1dbd65b5fd7061f8e787
Author:     Johannes Berg <johannes.berg@intel.com>
AuthorDate: Thu, 11 Aug 2016 11:50:21 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 18 Aug 2016 15:36:13 +0200

Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"

This reverts commit:

  fa7d81bb3c269 ("drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference")

As Peter explained:

  [...] lockless_dereference() is _stronger_ than READ_ONCE(), not weaker.

  [...]

  Also, clue is in the name: 'dereference', you don't actually dereference
  the pointer here, only load it.

My next patch breaks the compile without this revert, because it assumes
you want to deference and thus also need the struct type visible (which
it isn't here), so revert it.

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1470909022-687-1-git-send-email-johannes@sipsolutions.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 drivers/gpu/drm/drm_fb_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index ce54e98..0a06f91 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -464,7 +464,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
 
 	/* Sometimes user space wants everything disabled, so don't steal the
 	 * display if there's a master. */
-	if (lockless_dereference(dev->master))
+	if (READ_ONCE(dev->master))
 		return false;
 
 	drm_for_each_crtc(crtc, dev) {

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[PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference" Johannes Berg <johannes@sipsolutions.net> - 2016-08-11 12:00 +0200
  [PATCH 2/2] locking/barriers: suppress sparse warnings in lockless_dereference() Johannes Berg <johannes@sipsolutions.net> - 2016-08-11 12:00 +0200
    [tip:locking/core] locking/barriers: Suppress sparse warnings in  lockless_dereference() tip-bot for Johannes Berg <tipbot@zytor.com> - 2016-08-18 13:10 +0200
    [tip:locking/urgent] locking/barriers: Suppress sparse warnings in  lockless_dereference() tip-bot for Johannes Berg <tipbot@zytor.com> - 2016-08-18 15:50 +0200
  Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to  lockless_dereference" Daniel Vetter <daniel@ffwll.ch> - 2016-08-11 12:40 +0200
    Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to  lockless_dereference" "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-12 00:30 +0200
      Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to  lockless_dereference" Johannes Berg <johannes@sipsolutions.net> - 2016-08-12 08:10 +0200
      Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to  lockless_dereference" Peter Zijlstra <peterz@infradead.org> - 2016-08-12 20:30 +0200
        Re: [PATCH 1/2] Revert "drm/fb-helper: Reduce READ_ONCE(master) to  lockless_dereference" "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-12 21:20 +0200
  [tip:locking/core] Revert "drm/fb-helper: Reduce READ_ONCE(master)  to lockless_dereference" tip-bot for Johannes Berg <tipbot@zytor.com> - 2016-08-18 13:10 +0200
  [tip:locking/urgent] Revert "drm/fb-helper: Reduce  READ_ONCE(master) to lockless_dereference" tip-bot for Johannes Berg <tipbot@zytor.com> - 2016-08-18 15:50 +0200

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